You don't have to physically take the button out to replace the LED. The power LED pulls easily out of the holder it's in - there's just a tiny bit of hot glue holding it in.
I've replaced the blue LED on my Define Mini and Define XL with green ones, and glued a red LED to the side of the clear plastic surround to act as a HDD LED. Works perfectly - the plastic glows green and when the HDD is accessed it flashes red. I just hot-glued the red LED to the side of the clear plastic, beside the green one.
Both LEDs are normal 3mm items.
Workstation: Ximatec Asgard, i5-3570S, Arctic i30, 16GB DDR3L-12800, Asus HD7850, 256GB Crucial M4, no case fans
NAS&Downloader: Asustor AS-202T, 2x Seagate 4TB ST4000DM000
HTPC1: Asus Essentio, i3-3220T, 8GB DDR3L-12800, 250GB Samsung 840, Windows 8.1 Enterprise
HTPC2: Akasa Cypher, DH61AG, Pentium G620, 4GB DDR3L-10666, Crucial M4 32GB mSATA, Win7